This invention relates generally to switched capacitor circuits and more particularly, it relates to a switched capacitor amplifier having a significantly reduced current in the switched capacitor stages. The invention has particular applications in band-pass filters, telecommunication equipment, speech synthesis circuits, analog-to-digital converters, and digital-to-analog converters.
In general, switched capacitor circuits have been preferred since capacitance circuits could be formed on the same integrated semiconductor chip having other circuit elements. In contrast, resistive network circuits are typically required to be external to the chip such as by being plated on a hybrid integrated circuit or by having resistors physically attached to printed circuit boards. This procedure is much more costly and time-consuming. However, by utilizing switched capacitor circuits having an operational amplifier there was suffered the disadvantage in that a high bias current on the operational amplifier was required. It would, therefore, be desirable to provide a switched capacitor circuit wherein the operational amplifier is operated in a more efficient and effective manner.
Although prior art operational amplifiers are typically current biased at a constant high level, I have found that the operational amplifier only needs the high current bias at the beginning of each cycle of operation and thereafter it can use much less bias current. To this end, the present invention provides a switched capacitor amplifier wherein the operational amplifier is supplied with a high bias current only at the beginning of each pulse cycle when a high slew rate is required and thereafter is current biased at a relatively low constant level so as to reduce current consumption.